Shakespeare has a way of slipping through the fingers. After three and a half years' research, and the detailed examination of six paintings, the National Portrait Gallery has concluded that the so-called Chandos portrait shows the true face of Shakespeare - probably.

The gallery's Dr Tarnya Cooper said that the claim of the Chandos portrait to represent Shakespeare has "increased, but it's not absolutely watertight. We may never find the clincher piece of evidence - though it may yet turn up".

It has, she said, the strongest claim of any of the known contenders to be a true portrait of Shakespeare, six of which go on show today at the gallery's Searching for Shakespeare exhibition. The Chandos portrait was the first painting given to the gallery, in 1856.

"It would be lovely to be categorical," she added. "It is certainly fairly likely we are looking at the face of Shakespeare, but we'd need a document or a signature to prove it beyond all doubt."

The face in the painting, which dates from the right period, resembles that in the engraving by Martin Droeshout the Younger on the frontispiece of the First Folio - which was authenticated as a true likeness by Ben Jonson. Dr Cooper also said that the earring and the loose shirt-ties worn by the sitter point to a representation of a poet. Similar fashions were sported by John Donne and literary patron the Earl of Pembroke.

The provenance of the painting, though far from complete, is the best of any of the alternatives. According to an early 18th-century source it was owned by Sir William Davenant, who revered Shakespeare, but was only 10 when he died. It is impossible entirely to discount, said Dr Cooper, that Sir William was making a "wishful assumption".

Of the other portraits on show, one, owned by the Royal Shakespeare Company, has been shown to be a 19th century fake. Technical analysis by the gallery last year found that a yellow pigment in the Flower portrait was not on the market until 1814.

Two others, the Sanders portrait and the Grafton portrait, were thrown out because their claims were based on purely circumstantial evidence. The sitter of the Grafton portrait was the same age as Shakespeare, according to its inscription, but is unlikely to be him. The Sanders portrait, also dated, shows a man much younger than the 39 years Shakespeare had reached by 1603.

The Souest portrait, while meant to show what the poet might have looked like, is half a century too late, painted during the Restoration. Finally, the Janssen portrait had, it was shown during conservation work in 1988, been painted over to make the sitter look balder, and more "Shakespearean". It had also been given a fake inscription.

Sensitive

The Chandos portrait, of a sensitive, almond-eyed fellow with a gold hoop in his left ear and a receding head of hair, has not always been regarded as having quite the look appropriate for England's national poet.

In 1864, one critic, J Hain Friswell, wrote: "One cannot readily imagine our essentially English Shakespeare to have been a dark, heavy man, with a foreign expression, of decidedly Jewish physiognomy, thin curly hair [and] a somewhat lubricious mouth" - an unpleasant xenophobic fantasy, but revealing, perhaps, of an ancestral urge for the national poet not only to have an identifiable face, but look the part.

The 1623 Droeshout engraving, while it has authenticity on its side, hardly constitutes great art. The head seems to float uncomfortably above the collar, while the doublet is ineptly managed. Nor was it made during the poet's lifetime: he died in 1616.

In the end, it is unlikely that the claim of the Chandos portrait will ever be proved beyond doubt. For many, it is of little account. As Ben Jonson urged in his preface to the First Folio of Shakespeare's plays: "Reader, looke not on his Picture, but his Booke."